Boolean logic simply means using the words AND, OR, and NOT to limit or expand your search.
When you combine two words with AND, the search engine will show you results that include both terms. For example, if I searched - corn AND ethanol - each result will have both corn and ethanol in them.
OR, on the other hand, is a great way to search for synonyms, because it shows you results with either term.
The third boolean operator is NOT. NOT is a great way to limit your search by excluding terms you don't want to see. For instance, a search for - corn NOT ethanol - will bring back results that contain corn but eliminate results that include ethanol.